The invention concerns a portable infusion unit for the automatic discharge of liquid from an exchangeable injection ampule.
Injection ampules are destined, in contrast to customary ampules, for use as a syringe and, for this purpose, they are provided with a tightly closing piston on one end and open for the introduction of a piston rod and are equipped with an outlet piece on the other end. Injection ampules are also called "carpules".
An infusion unit of this kind is known from the published European Patent Application No. 42 282. The injection ampules to be used with the known unit have a flange at the rear end which is placed into grooves on opposite casing walls of the unit. The piston rod inserted in the piston of the injection ampule is placed on a pinion of the driving device arranged on the bottom of the casing between the walls located opposite each other and is pressed on the pinion by means of a pressing-down element so that it can advance the piston rod which consists of plastic and is smooth in longitudinal direction. In comparison with other units of this kind, for example, the one known from the U.S. Pat. No. 3,886,928, where a gear rack is arranged in the casing which is driven by a pinion of the driving device and is to be connected to the piston rod, the unit has the advantage that the connecting operation is eliminated and, therefore, the unit equipped with the injection ampule is shorter. The disadvantage which exists already in principle with regard to the known units of this kind, i.e. the fact that a high torque is required for the advance of the rod and the risk is run that the rod is not reliably advanced or even runs back owing to the comparatively high counter-pressure, exists even to a higher degree in the case of the unit known from the European Patent Application: Since the pinion does not engage in a gear rack but in a smooth rack, there is the additional risk of slippage whereby the rack may even slide back over the pinion.